1. Field of the Invention
The present invention relates to a method and apparatus for detecting a frame boundary from a received signal in a digital communication system.
2. Description of Related Art
A digital communication system can transmit/receive one of a discrete number of waves in an electrical or an optical form, for a discrete time interval, and includes a system transmitting data in which information represented as digital data or analog information are digitized.
As an amount of digital information increases, demands for a system which can quickly and reliably exchange digital information are also being increased. Also, in comparison to a conventional analog communication system, a digital communication system has advantages in that information can be easily retrieved and system is easily embodied. Thus, digital communication system is being widely utilized for commercial and military purposes. Accordingly, today's digital communication systems can be widely applied to various fields, such as digital broadcasting, wired/wireless Internet, digital optical communications, satellite communications, and digital mobile communications.
Most digital communication systems divide digital data to be transmitted, that is, a series of bit streams into a certain size of a frame unit. A receiver side receives a signal which is in the form of consecutive frames, and detects a frame boundary so as to extract exact data from the received signal. In this instance, the frame boundary indicates a starting position of the frame or an ending position thereof. An accurate frame boundary detection is an important factor in a multi-path wireless communication system where a channel delay is severe.
FIG. 1 illustrates an example of an Orthogonal Frequency Division Multiple Access (OFDMA) Time Division Duplex (TDD) frame structure. The frame structure illustrated in FIG. 1 follows an Institute of Electrical and Electronics Engineers (IEEE) 802.16d/e standard. As illustrated in FIG. 1, a downlink (DL) frame is a connection between a mobile base station, i.e., a radio access station (RAS), and a portable device, i.e., a portable subscriber station (PSS). Also, the DL frame includes a DL preamble, a DL subframe and an uplink (UL) subframe. Hereinafter, a “preamble” as used in the present specification indicates a DL preamble included in a DL frame.
Referring to FIG. 1, a preamble is allocated to a first symbol of a DL frame. A starting position of the preamble may be identified on the basis of a frame boundary detected according to an aspect of the present invention. The preamble of which the starting position is identified is utilized for estimating a carrier frequency offset and searching for a cell containing a portable device. Also, the preamble may be utilized for restoring a certain data symbol contained in the frame.
OFDM/OFDMA signal is transmitted as a symbol unit. In the case of OFDM/OFDMA symbols consecutively transmitted in a time domain, signals may be overlapped or distorted because of a symbol delay occurring in a multi-path channel. To prevent the distortion of a signal caused by the symbol delay, a guard interval is inserted between consecutive symbols. In this instance, the guard interval is longer than a maximum delay spread. An OFDM/OFDMA symbol period includes a guard interval and a valid symbol period including data to be transmitted. A receiving end receives the transmitted symbol inserted between the guard intervals, as described above. The receiving end removes the guard interval and extracts data corresponding to a valid symbol period from the symbol. After this, the receiving end demodulates the data. In order to prevent a distortion of orthogonality which may occur due to a delay of a subcarrier, a part of a signal corresponding to a last portion of a valid symbol interval is copied and inserted before the symbol in the guard interval. This is referred to as a cyclic prefix (CP). As described above, since a preamble is also a kind of an OFDM/OFDMA symbol, CP is inserted in front of the preamble symbol.
FIG. 2 is a diagram illustrating a segment-wise preamble transmission structure according to an IEEE 802.16d/e standard. As illustrated in FIG. 2, preamble subcarriers for each segment are arranged at a predetermined interval, for example, at 3 subcarrier intervals in FIG. 2. As well known to those skilled in the art, transmission data of an OFDM/OFDMA system is converted to a time domain signal via an inverse fast Fourier transform (IFFT) module of a transmitting end, and the converted signal is transmitted. The above-described operation is applied to preambles constituting a first symbol in a frame. Due to a frequency shift property of an FFT/IFFT, a preamble having a transmission structure as shown in FIG. 2 is received in the form of a signal which is periodically repeated in a time domain.
As an example, when a number of segments is three as illustrated in FIG. 2, a preamble signal in a time domain includes three repetition lengths as illustrated in FIG. three. As described above, a received preamble signal has a property of being repeated once per the repetition length. Accordingly, a frame boundary may be detected by correlating a received signal and a signal obtained by delaying the received signal by a predetermined period, for example, as much as the repetition length.
However, when an FFT size is not a multiple of three, each repetition length may be represented as a non-integer value. As an example, as illustrated in FIG. 3, each repetition length may be represented as 1024/3=341.33 samples. In this instance, FIG. 3 is a diagram illustrating a configuration of a preamble signal when an FFT size is 1024 samples and a number of segments is three according to an IEEE 802.16d/e standard. As described above, a digital signal consists of discrete samples. Accordingly, when a repetition length is represented as a non-integer value, an error may occur in frame boundary detection.
The frame boundary detecting apparatus may experience a peak broadening effect by a repetition length having a non-integer value as its length. In this instance, the peak broadening effect indicates that a correlation of a received signal and a delayed signal obtained by delaying the received signal by a predetermined time forms a wide peak group, not a sharp peak around a frame boundary index, with respect to a series of consecutively received signals. The peak broadening effect may decrease the accuracy of frame boundary detection.
To prevent the effect of an error occurring as described above, oversampling three times is required. However, oversampling three times also increases a correlation length by three times. Also, since three conjugate operations, three multiplication operations, etc., are needed, a correlation calculation complexity is increased by three. An increase in the correlation calculation complexity indicates that hardware and software resources necessary for embodying a frame boundary detecting method and apparatus are also increased.
Also, in the conventional frame boundary detecting method, a performance may be significantly deteriorated when a threshold is precisely set. FIG. 4 is a flowchart illustrating an example of a conventional frame boundary detecting method. Referring to FIG. 4, in the conventional frame boundary detecting method, in operation 410, a delayed signal is generated by delaying a received signal by a predetermined period. Also, in operations 420 and 430, an auto-correlation value P(n) and a power R(n) of a received signal are calculated by using the received signal and the delayed signal. In operation 440, a comparison value M(n) in which the P(n) is normalized to the R(n) is calculated. Finally, in operation 450, the calculated M(n) and predetermined threshold T2 are compared so as to determine a frame boundary.
Namely, according to the conventional frame boundary detecting method, a frame boundary is determined by comparing a normalized auto-correlation value and a predetermined threshold. Accordingly, the threshold must be very precisely set. If the threshold is not properly set, more than a single frame boundary index are obtained. This will deteriorate reliability of frame boundary detection.
In other words, the conventional frame boundary detecting method has disadvantages such as an increase in complexity of a receiving end and deterioration of detection performance. The above-described disadvantages act as factors to limit a low power consumption and a highly efficient design of a receiving end. These properties may be not suitable for a frame boundary detecting method for a portable device of a wireless communication system which is currently in the spotlight.
Accordingly, the present invention suggests a new technology which can detect a frame boundary from a received signal with greater accuracy while using a small amount of resources.